The harmful effects from exposure to ultraviolet (UV) radiation can be classified as acute or chronic. The acute effects of UV-A and UV-B exposure are both short-lived and reversible. These effects include mainly sunburn (or erythema) and tanning (or pigment darkening). The chronic effects of UV exposure can be much more serious, even life threatening, and include premature aging of the skin, suppression of the immune system, damage to the eyes, and skin cancer.
Sunburn (or erythema) is redness of the skin, which is due to increased blood flow in the skin caused by dilatation of the superficial blood vessels in the dermis as a result of exposure to UV radiation. High UV doses may also results in edema, pain, blistering, and peeling of the skin a few days following exposure. UV-B radiation is believed to be mainly responsible for sunburn as it is more erythmogenic by a factor of 1,000, however since there is more UV-A radiation reaching the earth’s surface, UV-A contributes 15-20% to the sunburn reaction in the summer months. Risk factors for sunburn include fair skin, red or blond hair, blue eyes, and freckles. For people with fair skin, it takes only 15-30 minutes in midday sun to induce erythema. In terms of areas of the body that are more susceptible to sunburn, the face, neck, and trunk are two to four times more sensitive than the limbs. In addition, children and the elderly are believed to be more sensitive to UV radiation and may burn more easily. A sunburn reaches its maximum redness eight to 12 hours after exposure and fades within one to two days (1).
Tanning refers to delayed pigmentation of the skin, or melanin pigmentation. It usually becomes noticeable one to two days after exposure to the sun and gradually increases for several days persisting for weeks or months. Tanning results from an increase in the number of functions melanocytes (pigment cells) resulting in increased activity of the enzyme tyrosinase. This leads to the formation of new melanin and an increase in the number of melanin granules throughout the epidermis (2). Tanned skin need not only be considered a harmful effect as it does confer some protection for subsequent exposure to the sun, but the degree of protection is thought to be moderate and not sufficient as a sunscreen for Caucasian skin. However, there is another mechanism that may provide more protection for subsequent exposures. In addition to tanning and sunburn, thickening (or hyperplasic) of the epidermis also occurs and is thought to be a significant component of a mild sunburn reaction. A single moderate exposure to UV-B radiation is sufficient to induce 3-fold thickening in the stratum cornea that lasts one to two months. This thickening is likely to be more important than tanning for providing endogenous photoprotection for those with Caucasian skin (1).